Use consistent capitalization for "Add New" in post type scaffold to mat...

A very small update to the `wp scaffold post-type` mustache template that capitalizes "Add new" to "Add New", keeping the UI consistent with built-in post type labels.

![add-new-capitalization](https://f.cloud.github.com/assets/1149744/1525460/3a011284-4bd5-11e3-8c48-6bc5cf6fcf8f.png)